1. Field of the Invention
This invention relates to a developing device for use in an electrophotographic printer.
2. Description of the Prior Art
In the prior art, Japanese Patent Publication No. 59-8831 discloses a device characterized in that a fixed magnetic field generating means is disposed inside a moving toner support means, a magnetic member for regulating the layer thickness of magnetic toner is disposed outside the toner support means, i.e. on the opposite side of the magnetic field generating means, and a stationary magnetic field is generated between the fixed magnetic field generating means and the magnetic member as to spread across the toner support means (sleeve). In this device, the magnetic toner expected to stay in the stationary magnetic field is drawn out in response to the movement of the toner support means, so that a thin layer of magnetic toner is formed on the surface of the toner support means.
Japanese Patent Publication No. 61-48157 discloses a device comprising a developing magnetic pole corresponding to the foregoing fixed magnetic field generating means, a member corresponding to the foregoing toner support means, a means for supplying toner onto the periphery of the member, a doctor blade for controlling the amount of toner being supplied, and a counter magnetic pole plate made of magnetic material disposed between the doctor blade and the developing zone, wherein a curtain defined by magnetic lines of force is formed between the developing magnetic pole and the counter magnetic pole plate.
Further, Japanese Patent Publication No. 63-789 discloses a device characterized in that the layer thickness of toner is regulated by a rigid blade disposed in confronting relation to the toner support means.
In the first prior device, since the stationary magnetic field generated by the fixed magnetic field generating means and the magnetic member is strong, the magnetic toner is readily conveyed up to the stationary magnetic field in response to the movement of the toner support means; but, only a little amount of magnetic toner can be drawn out from the stationary magnetic field because the force of staying there is strong. Therefore, although this device is advantageous in forming a thin layer of magnetic toner, the amount of magnetic toner staying in the stationary magnetic field increases, resulting in an aggregation of magnetic toner. In this state, if the magnetic toner is drawn out, the density of the magnetic toner thus drawn out fluctuates, making printed characters nonuniform.
Further, since the stationary magnetic field is generated by the fixed magnetic field generating means and the magnetic member, the distance between them must be adjusted finely, making device assembly difficult.
In the second prior device, since the gap between the doctor blade and the sleeve is narrow, if foreign matter such as dust included in the toner gets in the gap and causes clogging, such a clogged portion prevents supply of the toner. As a result, a portion of the surface of the sleeve bears no toner, resulting in a corresponding blank stripe on a developed picture.
Further, to form the curtain of magnetic line of force between the developing magnetic pole and the counter magnetic pole plate, the distance between them must be adjusted finely, making assembly difficult.
In the third prior device, the layer thickness of the toner is influenced by the width and surface roughness of the end face of the blade which faces the sleeve.